Original Paper www.karger.com/cpb 319 Harl/Schmölzer/Jakab/Ritter/Kerschbaum: Accepted: February 07, 2013 Chloride Channel 1421-9778/13/0313-0319$38.00/0 Blockers Suppress This is an Open Access article licensed under the terms of the Creative Commons Attribution- NonCommercial-NoDerivs 3.0 License (www.karger.com/oa-license), applicable to the online version of the article only. Distribution for non-commercial purposes only. Chloride Channel Blockers Suppress Formation of Engulfment Pseudopodia in Microglial Cells Barbara Harl a Judith Schmölzer a,b Martin Jakab b Markus Ritter b,c Hubert H. Kerschbaum a a Department of Cell Biology, University of Salzburg, Salzburg; b Institute of Physiology and Pathophysiology, Paracelsus Medical University, Salzburg; c Gastein Research Institute, Paracelsus Medical University, Salzburg Key Words Cl - -conductance Abstract Background/Aims: depends on the formation of engulfment pseudopodia surrounding the target. We tested in microglia, monocyte-derived cells in the brain, whether a swelling-activated Cl - -current (I Cl, swell also contributes to local expansion and retraction of engulfment pseudopodia. Methods: We used scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM) to visualize and quantify the uptake of polystyrene microbeads (MBs) by microglial cells. Flow cytometry was used for cell volume measurments and I Cl, swell was measured by whole-cell patch clamp. Results: Cl - -free extracellular solution attenuated MB uptake and that the Cl - -channel blockers DIOA, microglial cells. Microglial cells exposed to MBs in the presence of Cl - channel blockers failed to extend engulfment pseudopodia. We observed that cells containing at least three MBs revealed an about twofold increase in current density of I Cl, swell compared to cells without MB. phagocytosis. Pre-conditioning of cells in hypo- or hypertonic medium for 12 16 hours caused a decrease in MB uptake. Conclusion: I Cl, swell contributes to formation of engulfment pseudopodia and participates in engulfment and particle uptake in microglial cells. Copyright Hubert Kerschbaum University of Salzburg, Department of Cell Biology, Hellbrunnerstraße 34, 5020 Salzburg (Austria) E-Mail hubert.kerschbaum@sbg.ac.at
www.karger.com/cpb 320 Introduction Anion-conductance activation is one of the earliest events in the innate immune responses. Recognition of opsonised Candida albicans by human polymorphonuclear - in these cells [1]. Accordingly, - -channel activity in human PMNs decreases the uptake of opsonized zymosan - -conductance in rodent microglia, monocyte-derived cells in the nervous system, suppresses phagocytosis of E. coli [3] or engulfment of microspheres (microbeads, MBs) [4]. In contrast to uptake of particles, blockade of chloride intracellular ion channel 1 ( amyloid-beta (A ) uptake in microglia and macrophages [5]. - -channels have - - 3 are localized in the membrane of phagolysosomes in phagocytes [2, 6, 7]. Phagolysosomes organelles (phagolysosomes) are crucially involved in phagocytosis and killing of pathogens. - -conductance in the - -conductance - -conductance is a particle, engulfment pseudopodia may be envisioned as local subcellular volume changes. Similar to global local cell swelling or shrinkage, respectively. In such - - digestion lead to an increase in osmotic pressure, which needs to be counterbalanced by + - - channels [15 17]. Similarly, in phagocytes the uptake of particles could promote an increase - -conductance to prevent deleterious osmotic stress. Accordingly, an increase in the - -current (I ) cells to osmotic challenges prior to application of particles will modulate the phagocytotic capacity. cascades induced by the MB uptake with those elicited by biological material, like pathogen associated molecular patterns - as well as - I on MB uptake.
www.karger.com/cpb 321 Materials and Methods Cell culture 2 2, primary according to Austrian law and approved by local authorities. 2 2 week cells were split and supplied with new medium. Reagents and solutions - -channel blocker H - + - -cotransport inhibitor R(+)[(2-n-butyl-6,7-dichloro-2- H - 2 2 - - 4 concentration of ionized calcium in this solution was 0.31 mm. Phosphate buffered saline (PBS) contained 2 4 2 4 2 atmosphere of the 2 Flow cytometry Cell volume (CV) regulation 5 cells in 2
www.karger.com/cpb 322 Scanning electron microscopy 4 2. incubated in 6 2 Agar Stereoscan 250 scanning electron microscope. Electron micrographs were digitized and analysed via - 6-2. After incubation times further processing followed as described above. 2 6 MBs conditions in the presence of MBs, (iii) for 1 h in isotonic saline containing MBs, and (iv) for 15 min in isotonic saline followed by 1 h in isotonic conditions in the presence of MBs. After different incubation Confocal laser scanning microscopy (CLSM) 4 6 2. After the incubation, cells were washed = Determination of successful particle binding, engulfment and internalisation numerically. Successful MB engulfment and internalization was detected by microglia whole particle particle internalization was observed when the MBs were in the same optical plane as the nuclei. Electrophysiology - -currents were monitored in the standard whole-cell recording
www.karger.com/cpb 323 2 2 I 2 2 - capacitances were compensated using an electronic feedback via - -current was monitored either Statistical analysis All data are presented as mean ± standard error of the mean (s.e.m.). Statistical analysis was performed P < 0.05 and P < 0.01. All electrophysiology data are presented as mean ± s.e.m. Statistical analyses (t-tests) Results Cl - -channel blockers suppress I Cl, swell, uptake of microbeads, and formation of engulfment pseudopodia cells with a hypertonic intracellular solution activated an outwardly rectifying current in I - IC 50 of 4 took place all over the cell body and was not restricted to the lamellipodium. - - - uptake was unaffected after 15 min, but attenuated by 40% after 1 h. on the incubation time. After 15 min and 1 h the relative number of MB containing cells was
www.karger.com/cpb 324 Fig. 1. swell was evoked either by superfusion of a cell with (A) hypotonic solution (85% of versus versus - were used and currents were monitored during direction of I data point). Fig. 2. Engulfment and attachment of hydrophobic polystyrene MBs by primary microglia and microglial cell cell with engulfed MBs at the soma and at the lamellipodia and attached engulfed MBs at the soma of the cell and an attached sphere at the lamellipodia. Fig. 3. - -free solution inhibited phagocytosis of MBs by the microglial cell mean ± s.e.m.. Asterisks (**) indicate P < 0.01, when compared with control cells. 53.2 ± 2.5% did not contain a single MB, 28.0 ± 1.1% contained one MB, 12.6 ± 1.5%
www.karger.com/cpb 325 Fig. 4. Blockers of I inhibit phagocytosis of MBs by primary microglial cells (primm) and the microglial of MBs engulfed by primm after 15 min of incubation in the absence and presence of inhibitors, respectively. respectively, when compared with control cells or between two groups as marked by a line. contained two MBs, and 6.2 ± 0.2% contained more than two MBs. Incubation with 200 had engulfed more than two MBs.
www.karger.com/cpb 326 Fig. 5. + - - -channel blocker NPPB inhibited phagocytosis of represent number of cells without phagocytosis (white columns), cells having successfully engulfed one MB (light grey columns), two MBs (black columns), and more than two MBs (dark grey columns). Number of control conditions. remaining 2% is distributed among cells containing one and two MBs. conditions the overall number of primary microglial cells containing MBs was 64.5 ± 4.6% At the single cell level only 35.5 ± 4.6% of primary microglial cells did not contain a single MB after 15 min of incubation. 27.5 10% for cells containing one MB and of cells ( led to signs of apoptosis (see below), we tested its effect at the IC 50 for I Inhibition of particle engulfment as well as block of I showed similarities in their and effectively 50% compared to control conditions and the IC 50 for
www.karger.com/cpb 327 Fig. 6. represent the relative number of cells without MBs (white columns), cells having successfully engulfed one as mean ± s.e.m.. Asterisks (*, **) indicate P < 0.05, P < 0.01, respectively, when compared with control conditions or between two groups marked by a line. I is at concentrations of of cells able to phagocytose more than two beads can be observed at an NPPB concentration - -currents with biophysical and pharmacological properties similar to I 2+ or camp in contrast
www.karger.com/cpb 328 Fig. 7. - increase in cells containing two MBs when compared between 15 min and 1 h, respectively - 2+ or camp, - -conductance, are also involved in particle uptake. - - channel blockers was due to inhibition of engulfment or due to less accessibility to MBs, like, e.g., by a decrease in cell migration. We assumed that no change or even an increase in the number of attached MBs, but a decrease in the number of engulfed MBs, indicated 6 2 an area with a diameter - -channel blockers had more particles attached than engulfed. MBs, however, on average 1.6 MBs were attached to the cell surface. support the assumption that - -channel blockers interfere with the engulfment process and - -conductance inhibition does not prevent MB - - - -channel
www.karger.com/cpb 329 Fig. 8. (A, B) Representative showing blebbing, a hallmark of apoptosis visualized by SEM. Arrows point to cells with blebbing.. Fig. 9. Scanning electron cells with successive engulfment of a hydrophobic polystyrene MB is attached to the soma of a microglial cell. (B) Numerous cell surface at the contact area between the microglial cell and the MB Protrusions augment on the base continuously, fusing together to an Simultaneously internalization completely enclosed by the cell membrane. At the area of process Engulfment and internalization is - - - - - 3- Cl - -channel blockers suppress formation of engulfment pseudopodia As mentioned above, pseudopodia formation and the following particle engulfment take place all over the cell body and are not restricted to the lamellipodium. transformed to
www.karger.com/cpb 330 Fig. 10. Scanning electron showing successive engulfment of hydrophobic polystyrene MB at the is attached to a lamellipodia of a microglial cell. (B) from the cell surface contact the MB, area between the cell and the MB. the whole MB is engulfed by the cell. be seen on the lamellipodia of the Fig. 11. Attachment of hydrophobic cell. Scanning electron microscopy of particle, overlapped at the area of contact and fused with each other. In contrast to mode - -channel blockers behaved differently - - - -, but are also permeable for other anions.
www.karger.com/cpb 331 Fig. 12. versus an of a cell containing at least three MBs (cell with MB, closed ( MB. (E) M MBs Particle uptake causes an increase in Cl - -conductance In hepatocytes, glucagon-induced catabolic processes are associated with an increase - I in microglial cells, volume if non-digestible particles accumulate within the cell. In our study, uptake of each 3 I
www.karger.com/cpb 332 Fig. 13. (E) solution are shown as columns representing the number of cells without MBs (white columns), cells successfully engulfed 1 MB (light grey columns), 2 MBs (black columns), and more than 2 MBs (dark grey when compared with appropriate control groups. slope of inward and outward current activation was steeper and the current density after I in control I - - I
www.karger.com/cpb 333 Pre-conditioning of BV-2 cells in hypotonic or hypertonic medium attenuates microsphere uptake If the observed up-regulation of I an increase in intracellular osmotic pressure following degradation of a digestible particle, long-term osmotic challenges might affect phagocytotic capacity. Accordingly, we assumed I h in hypotonic and hypertonic conditions, respectively, revealed a decrease in phagocytosis decrease by about 21% after decrease in MB uptake was visualized by an increase in the number of cells without MBs and a decrease in the number of Discussion - -conductance in an early phase of phagocytosis as well - -conductance is essential - -conductance may be related to similarities between phagocytosis and osmoregulation. Both rely on the activation - -conductance and osmotic challenges modulate particle uptake. Early consequences of I Cl, swell activation on phagocytosis a potential target is the formation of an engulfment pseudopodium. In the present study, - conductance in phagocytes following contact with a particle are a common phenomenon - + -ions, respectively - - + - leaves the cell. Similarly, in human PMNs, phagocytosis of opsonized Candida albicans - - -channel blockers suppresses engulfment of polystyrene MBs, E. coli, S. aureus, or C. albicans [2 4, and present study]. phagocytosis leads to cell swelling, we focused in the present study on swelling activated - -current (I ). Activation of I is due to an osmotic imbalance, like hypotonic - -conductance with a similar electrophysiological and pharmacological signature is also activated in normotonic conditions via 2+
www.karger.com/cpb 334 stimulation of contact between a particle and a phagocyte could trigger an intracellular signalling cascade, - -channels in microglial cells, even at normotonic conditions. We found that formation of engulfment pseudopodia is prevented by blockade of I important function of I activation in the early phase of phagocytosis is its contribution prevents uptake of particles. I - half of I IC 50 for this IC 75 for NPPB of I - -channels. - -dependent - - - or osmotic pressure (see below). morphological and structural similarities, which may be rooted to similarities in their - channels may contribute ion pathways + - 2+ + + - - 3- + + - -cotransporters are sorted to the leading edge. Because blockers of I - - -free conditions affects several - -dependent transport proteins like [22, 38], the contribution of these different ion channels and transporters to engulfment pseudopodium formation remains to be characterized. Since all blockers used - - In addition, these ion transport mechanisms are tightly linked to the cytoskeleton and hence regulate the dynamics for adhesion and pseudopodia protrusion [40 42]. Whether a similar polarized distribution of ion channels and transporters is established during the formation of engulfment pseudopodia is, however, not known yet. Late consequences of I Cl, swell on phagocytosis volume osmotic pressure following degradation of phagocytosed particles. We assume that phagocytosis promotes long-term activation of I to counterbalance increases in intracellular osmolytes due to degradation and metabolism of phagocytosed material by camp-mediated pathway not only promotes glycogenolysis and gluconeogenesis, but also cell shrinkage, whereas insulin not only facilitates glucose uptake and glycogenesis but also - -channels in hepatocytes via a camp- Epac-dependent signalling pathway [15]. Similarly to hepatocytes, digestion of engulfed material by phagocytes may increase the intracellular osmotic pressure, which causes an
www.karger.com/cpb 335 plasma membrane. In the present study we found that uptake of MBs causes an increase in I membrane. change in I to the the phagocytotic capacity is reduced after both hyper- and hypotonic preconditioning. anisotonic stress down-regulates processes, which pose further osmotic burden on it, like e.g. phagocytosis. that volume regulation due to I Since ion channels and transporters are linked to the actin cytoskeleton [45], ion channels in the plasma membrane and the cytoskeleton below the plasma membrane may cooperate to engulf particles. In conclusion, our observations indicate that phagocytotic uptake of polystyrene microspheres by microglial cells - -currents, because (i) - - - channel blockers as well as the uptake of microspheres, and (iii) engulfment of microspheres up-regulates I (iv). In addition, preconditioning of the cells to hypo- or hyperosmotic conditions decreases microsphere uptake. Abbreviations I - + - - + + SEM (scanning electron microscopy). Acknowledgements rd 2013.
www.karger.com/cpb 336 References - channels contribute to volume 2+ - -channels in rat 2+ microglial cells. + - - cotransport in + -
www.karger.com/cpb 337 - + - 2+ - of I 710.